Support partial stats at extremes on hash-sharded indexes and REGIONAL BY ROW tables

When collecting partial stats on specified columns, we search for an index prefixed by that column in:
https://github.com/cockroachdb/cockroach/blob/c64fb80786b82b0a848bf59e271d8b2ce44db674/pkg/sql/scan.go#L227

This doesn't work for hash-sharded indexes and REGIONAL BY ROW tables since partition keys prefix the indexes. We would also need to update `bounds.ConstructUsingExtremesSpans()` to build the correct spans to be scanned: https://github.com/cockroachdb/cockroach/blob/c64fb80786b82b0a848bf59e271d8b2ce44db674/pkg/sql/stats/bounds/extremes.go#L59

We would also need to update how we select columns to collect partial stats on when unspecified by selecting the first column after the partition key.

Jira issue: CRDB-41160